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Echidnas and platypuses are unusual mammals in that they lay eggs. As with most marsupials, the baby is very underdeveloped and spends much time in its mother's pouch while it matures.

One of the distinguishing characteristics of birds is that they lay eggs. As far as scientists know, no bird gives birth to live young. The ostrich lays the largest egg, which can weigh 3 pounds and measure 6 inches long and 5 inches wide. Most birds build nests, some of which can be intricate.

Cobras are unusual snakes because the female builds a nest and guards her eggs. She may have up to 40 eggs in her nest. However, when the eggs hatch, she leaves them behind.

Crocodile mothers also build nests and guard them. The heat within the nest determines the sex of the babies. If the temperature is less than 86 degrees Fahrenheit, most of the babies are born female. It it's above 90 degrees F, most are born male. Unlike the cobra, the crocodile continues to care for her babies for a while after they hatch.